Searches for hidden sectors using $K^{+}\to {\pi }^{+}X$ decays

Results from the study of the rare decays $K^{+}\to {\pi }^{+}\nu \bar{\nu }$, $K^{+}\to {\pi }^{+}{\mu }^{+}{\mu }^{-}$ and $K^{+}\to {\pi }^{+}\gamma \gamma$ at the NA62 experiment at CERN are interpreted in terms of improved limits for $\mathrm{B}(K^{+}\to {\pi }^{+}X)$ and coupling parameters of hidden-sector models, where $X$ is a mediator. World-leading limits are achieved for dark photon, dark scalar and axion-like particle models.